Calculating Fiber Optic Power Budgets

After a fiber optic cable plant is installed, it may be used with several different types of fiber optic networks. Computer networks, telephone signals, video links, and even audio can be sent on the installed fibers.

Each network type has a performance requirement for the fiber optic cable link. Most simply specify the maximum loss in the link that can be tolerated, a function of component specifications and installation quality. Others also specify the bandwidth performance of the fiber, which is determined by the specifications of the fiber chosen.

Every fiber optic link has a maximum loss of a link over which it can work. That loss is determined by the output power of the transmitter coupled into the fiber and the sensitivity of the receiver, all expressed in dB. For example, if the transmitter has an output of -20 dBm ( 20 dB down from a milliwatt or 10 microwatts) and the receiver has a sensitivity of -20 dB (30 dB down or 1 microwatt), the “link margin” is 10 dB. The loss of the fiber optic cable it uses must be less than 10 dB for proper operation.

While every link installed must meet some maximum loss to allow operation of the network intended to use it, different networks may have different link margins. Therefore, we use a different approach, considering link loss to be acceptable if it is less than standard maximum values calculated from the characteristics of the link installation.

Loss factors for fiber optic cable include the following:

• The fiber. Multimode fiber used in most premises links has a maximum loss of about 3.5 dB/km at 850 nm and 1.5 dB/km at 1,300 nm. Single-mode fiber, often installed in backbones for high-speed networks like Gigabit Ethernet, CATV, or telco links, has much lower loss, say 0.5 dB/km.

• Terminations. Connectors generally have losses of 0.2 to 0.5 dB per mated pair of connectors, but the industry standard to cover all types is 0.75 dB. A single connector has “no loss”—meaning that it’s usually measured with a power meter with a large detector that picks up all the light. Connectors are tested by mating an unknown connector to a good connector and measuring the loss of that mated pair.

• Splices. They are rare in multimode networks, but when used are generally mechanical splices expected to have losses of less than 0.3 dB. Single-mode fiber is usually spliced with a fusion splicer that welds the two fibers together in an electric arc, with much lower losses.

• Stress. Fiber optic cable pulled with too much tension during installation may be damaged. Each time you make a bend with a fiber optic cable, you put some stress in the fiber, which can cause loss. This loss will be higher at longer wavelengths (1,300 nm versus 850 nm in multimode and 1,550 nm versus 1,310 in single-mode.) Even cable ties tightened on the cable can cause loss. Stress loss should be zero!

Calculate the loss budget

To calculate the loss budget, determine the maximum loss for a normal installation. To begin, know the approximate length of the link and the number of connectors and splices.

For connectors, count the connectors on each end as one each (we’ll mate them to reference connectors when we test them) and each mated pair used as a patch connection in the cable link as one also.

Consider a simple link as an example:

• Link length: 0.3 km (~1,000 feet)

• Connectors: 3 (one on each end and one patch connection in the middle)

For the connector loss, 3 connectors x 0.75 dB = 2.25 dB. If we had splices, we would calculate the total loss the same way.

Adding the loss of the fiber to the termination losses gives us the total loss. Thus, our link should have a maximum loss at 850 nm of 1.05 dB + 2.25 dB = 3.30 dB. At 1,300 nm, the loss drops to 0.45 dB + 2.25 dB = 2.70 dB.

These become “pass/fail” numbers for testing. When you test the link, you should have less than a 3.3 dB loss at 850 nm and less than a 2.70 dB at 1,300 nm. If the loss exceeds that, you may have problems with the cable installation or the terminations, and must troubleshoot the installation. That’s a topic for another column. EC

HAYES is the founder of Fotec, the fiber optic test equipment company, and the Cable U training programs. He can be contacted at Jh@jimhayes.com.